Heat treatment of copper-chromium alloy steels



Patented Feb. 4, 1941 PATENT OFFICE HEAT TREATMENT OF COPPER-CHROMIUM ALLOY STEELS William Pollard Digby, London, England, assignor to Everard Tuxford Digby and himself, as partners No Drawing. Application June 28, 1937, Serial 8 Claims.

This invention relates to improvements in the treatment of alloy steels containing copper and chromium.

Various alloy steels are known containing copper in proportions of 1% or 2% and it has also been proposed to make alloy steels containing chromium and over 5% of copper. However, such steels containing over 5% of copper have not come into commercial use, as no satisfactory in method has been published for the fabrication of a satisfactory range of useful articles.

It is the object of the present invention to provide improved processes of manufacture of articles from such alloy steels. Further objects of If) the invention are to provide such alloys characterised by a novel structure and to provide worked articles of such alloys having novel physical properties.

Alloys containing over 5% of copper and over of chromium may be prepared in known manner, and they exhibit stain-resisting properties. If however, it is attempted to roll or forge such alloys at the usual temperatures employed for similar stain-resisting chromium alloys containing little or no copper; especia ly nickel chromium steels, it is found that the higher copper alloys are burnt, become cracked and exhibit the phenomenon of hot-shortness. Such undesirable results are preserved if it is attempted to forge the alloys at temperatures of 1400 to 1500 C., and even as low as 1200 C.

I have now discovered that these difficulties can be overcome if certain precautions are observed in mechanical working, though special methods of annealing and heat-treatment (for hardening) are also desirable, and these special methods of treating worked articles differ profoundly from those applicable to the well known nickel-chromium steels.

I have discovered that the alloys can be successfully worked for example by forging or rolling or drawing at an initial temperature of not much higher than 1000 C. and preferably between 800 and 900 C. For example, with an alloy containing 10% of copper and of chromium, the initial rolling temperature should not exceed 1000 C., although some result can be obtained at 1025 C. with extreme care and risk of wastage. Broadly speaking, the best range of initial working temperatures is from 800 to 900C.

Unlike alloy steels of similar composition, the copper-chromium steels can be mechanically worked as low as 700 C. or even 650 C. without danger of cold-shortness. However, for eco- In Great Britain July 3, 1936 nomic reasons it is obviously desirable to start at an initial temperature of 800 and preferably higher, otherwise too frequent re-heating would be necessary.

The invention further comprises certain meth- 6 ods of annealing and heat treatment as hereafter described in detail. Whereas nickel-chromium steels can either be annealed by quenching or hardened by slow cooling, the properties so obtained are not usually reversible, i. e. a soft 10 material cannot be converted into a hard one after the treatment. According to my invention, annealing is effected at a temperature of say '750 970 C., and is usually followed by hardening heat-treatment consisting of quenching or T5 air-cooling and the hard and soft properties so obtainable are reversible.

It has been found that when an ingot of the above alloy steel is obtained by slow cooling of .molten metal, the ingot-possesses a. dendritic structure, and it has further been found that if the mechanical Working (within the above described temperature limits) is performed continually in the same direction, it is possible to preserve the dendritic structure in the final article which then possesses a fine fibrous structure in which the individual long crystals are oriented, so as to be parallel to one another. This phenomenon is probably connected with the fibrous properties of copper and imparts great toughness and ductility to the alloy. To obtain this product by roll cogging care must be taken to avoid passing the material backwards and forwards between the same rolls since reversal of direction would destroy the parallelism of the long crystals.

In a casting the dendrites are initially radial. Rolling in one direction gradually causes them to lie as long very fine fibres, ultimately at right angles to their original direction, that is finally they will be parallel with the direction of rolling. If rolling is performed in a direction at right angles. this should be at an intermediate stage.

Similarly in hammer cogging the working must be always in one direction along the ingot.

If the molten metal is allowed to cool rapidly the crystalline structure is polyhedral. In this case the material can still be mechanically worked between the stated temperature limits, but the final product will not possess a fibrous structure and will not be suitable for certain purposes for which the fibrous material can be used.

In a preferred form of the present invention, a billet of a copper-chromium alloy steel is allowed to cool slowly to obtain a dendritic crystalline structure of long coarse crystals, whereupon the billet is passed to a soaking pit for re- ,heating and is mechanically treated, for instance, by hammer cogging the ingot from its centre to its ends, whilst the ingot is at a temperature from 800 to 900? C. A particularly suitable temperature for the cogging treatment has been found to be 850 C.

After this treatment, it has been found that the bar forged from the ingot or other mechanical element so forged, can be re-heated over a wide range; that is to say, from 900 C. to 1050 C. to allow of further mechanical treatment as by forging into a crank shaft or other mechanical article, or again, for rolling into strips or plate.

The forged or roller article may then be subjected to an annealing treatment, preferably from 750 to 970 C. varying with the percentages of the varying metals, for instance 910 C. has been found suitable for 15% chromium in an 8 to 10% copper chrome steel.

The lower temperatures are desirable for ease in pickling, while the higher temperatures are preferred if maximum softness is desired.

The finished article may be further treated by re-heating and either quenching in water or air cooling, the quenching temperature varying with the copper chromium content.

For .a contentof 15% chromium and 10% copper the temperature for maximum hardness is substantially 500 C.

-As illustrating the difference in properties ob- .tained by quenching from different temperatures,

I may mention that quenching from around the AC point (about 970 C.) gives a very hard alloy of high tensile strength, whilst quenching from around the AC point (between 500 and 600) gives steel of lesser hardness, slightly lower tensile strength but greater toughness.

The mechanical and heat treatments as set forth above are particularly applicable for chromium-copper-alloy steels in which the chromium content is analogous to that of the known nickel chromium steels; the corrosion-resistance increases progressively from to chromium or even 17-19% chromium, for commercial reasons, the upper limit of chromium content is about 20 to though higher percentages may be used.

The minimum copper content is about 5-6% and a preferred lower limit is '78%. The preferred range is 8-l0%. The copper may be as high as 20%, or even 40-50% but that is an obvious economic upper limit. Large increase in opper raises ductility at the expense of corroon-resistance. For deep-drawing alloys 15-25% of copper may be present.

The balance of .the alloy is principally and' almost entirely iron, though small quantities may be present of metals used as additions to steel, and in particular of metals which alloy with copper. The content of any such addition is preferably less than l-2 Nickel may be present as an accidental impurity (less than 1%) but it has little or no useful effect, and the nickel content should be less than 2%.,

The carbon content should be low; by sacrificing part of the stain-resistance, a. forgeable alloy may be made with up to 0.3% C but the C content is preferably less than 0.12%.

Copper chromium alloy steels subjected to a treatment as above, and having a chromium content of 15% or less will be found to be air hardening whilst those greater than 15% will not be air hardened.

It is preferable to vary the heat treatment with the percentage of chromium.

Chromium copper alloy steels treated in the manner of the present invention will be found to have high ductility, a high tensile strength and considerable resistance to corrosion forming stainless steels having similar properties to nickel chromium steel, and of a similar carbon content; that is to say, usually from 0.05 to 0.1% carbon.

The properties of the alloys can be varied within wide limits and the optima in certain di' actions, for particular properties (not necessari combined in the same sample) are as follows.

Firstly, the ratio of the yield point to the maximum stress may be between 70% and 85%.

Secondly, the izod impact values of the alloy may be from 50 to 120 ft. lbs. (forgings).

Thirdly, a percentage elongation of over (2 inch. test-piece) on a steel having a tensile strength of 30 English tons down to a steel having an elongation of 22% having a tensile strength of 52 English tons.

Fourthly, the alloys do not work-harden under machining. They can also be cold-rolled without appreciable hardening. Thus an annealed plate about 140 mils in thickness can be rolled down to a thickness of mils Without intermediate annealing and with an increase in the Brinell hardness from 190 to only 240.

I declare that what I claim is:

1. A process of producing articles of a steel alloy containing at least 5 percent copper, at least 10 percent chromium and the balance substantially all iron, which consists in allowing the molten alloy to cool slowly to obtain a dendritic structure, and mechanically working the solid metal at a temperature between 1000 degrees C. and 650 degrees C. and in a substantially unidirectional manner to preserve the orientation of the dendrites.

2. A process of producing articles of a steel alloy containing at least 5 percent copper, at least 10 percent chromium and the balance substantially all iron, which consists in allowing the molten alloy to cool slowly to obtain a dendritic structure, and mechanically working the solid metal substantially unidirectionally to preserve the orientation of the dendrites, said working being begun at a temperature between 900 and 1000 degrees C. and being concluded at a temperature of not less than 650 degrees C.

3. A process of producing articles of a steel alloy containing at least 5 percent copper, at least 10 percent chromium and the balance substantially all iron, which consists in allowing the molten alloy to cool slowly to obtain a dendritic structure, and mechanically working the solid metal at a temperature between 800 degrees C. and 900 degrees C. and in a substantially unidirectional manner to preserve the orientation of the dendrites.

4. A process of producing articles of a steel alloy containing at least 5 percent copper, at least 10 percent chromium and the balance substantially all iron, which consists in allowing the molten alloy to cool slowly to obtain a dendritic structure, mechanically working the solid metal at a temperature between 1000 degrees C. and 650 degrees C. and in a substantially unidirectional manner to preserve the orientation of the dendrites, and annealing the worked metal by maintaining it between 750 degrees C. and 970 degrees C.

5. A process of producing articles of an alloy containing to 25 percent copper, to 25 percent chromium, and the balance substantially all iron and containing less than 2 percent of elements alloyable with copper, and less than 0.3 percent carbon, which consists in cooling the molten metal slowly to obtain a dendritic structure, mechanically working within the range of temperatures between 650 degrees and 1000 degrees C. by substantially unidirectionally forging the same to preserve the orientation of the dendrites, annealing by maintaining at 750 degrees C. to 970 degrees C., and rapidly cooling from a temperature of substantially 500 degrees C. to 700 degrees C.

6. A process of producing articles of an alloy containing copper 5 to 20 percent, chromium 10 to 19 percent, less than 0.3 percent carbon, and the balance substantially all iron, which consists in slowly cooling an ingot thereof to produce a dendritic structure, ire-heating and mechanically working at a temperature substantially between 800 and 900 degrees C. to reduce the crosssection and therewith cause the crystals to be extended in length, said working being accomplished with always the same direction of advancement of the metal for preserving the dendritic structure and causing the crystals or fibers to lie substantially parallel in orientation along the length of the worked bar or plate, and concluding the working at a temperature of not less than 650 degrees C.

7. A process of producing articles of an alloy containing 5 to 25 percent copper, 10 to 25 percent chromium and the balance substantially all iron and having a dendritic form, which consists in mechanically working the metal at a temperature between 800 degrees C. and 900 degrees C. in a substantially unidirectional manner to preserve the orientation of dendritic structures, annealing the metal by maintaining it at a temperature between 750 degrees C. and 9'70 degrees C. and rapidly cooling from a temperature of substantially 500 degrees C. to 970 degrees C.

8. A process of producing articles of an alloy containing substantially 10 percent copper, percent chromium, and the balance substantially all iron and containing less than 2 percent of elements alloyable with copper and less than 0.1 percent carbon, which consists in slowly cooling the molten metal to obtain. a dendritic structure, mechanically working the metal at a temperature between 800 degrees C. and 900 degrees C. in a substantially unidirectional manner to preserve the orientation of the dendrites, annealing by maintaining at a temperature at substantially 910 degrees C., and rapidly cooling from a temperatureoi substantially 500 degrees C. to 600 degrees C.

WILLIAM POLLARD DIG BY. 

